PURPOSE: To evaluate the indication for intracavitary Urokinase(UK) in the treatment of Ioculated pleural effusion. MATERIALS AND METHODS: We analyzed CT and US in 31 patients who were treated with intracavitary UK in Ioculated pleural effusion. In each patient, a single chest catheter (10-12F) was insected under imaging guidance. When the amount of drainage was less than 100ml/day, UK was instilled through the catheter until less than 50ml/day was drained. On follow-up chest radiographs of more than 1 month, we classified the results of treatment into 3 groups:(1) completely effective (lung expansion, over 80%);(2) partially effective (20-80%); (3) ineffective (below 20%) group. Sonographic pattern of pleural fluid was classified into anechoic, septated, and honeycomb appearances and the thickness of parietal pleura was measured on CT. RESULTS: Sixteen patients were completely effective, nine were partially effective, and six were ineffective. patients with completely or partially effective outcome had anechoic and linear septated appearance on had less than 4mm of parietal pleural thickness on CT. Of six ineffective patients, US showed linear septated in one patient and honeycomb appearance in five patients and the thickness of parietal pleura on CT was 3 mm in one patient, 4ram in two patients, 5mm in one patient, and 6mm in two patients. CONCLUSION: UK instillation through percutaneous catheter was an effective method in the treatment of Ioculated pleural effusion. However, we found near complete reaccumulation of pleural fluid when honeycomb appearance of pleural fluid on US or more than 5mm parietal pleural thickness on CT was observed, which might suggest that we should consider the other kinds of treatment method in those patients.